The invention concerns power feed systems incorporated in telephone and/or information technology terminals connected to transmission lines which additionally feed them remotely with electrical power, for example S interface terminals connected by a passive bus to an integrated services digital network.
The conventional objective of such power feed systems is to limit to predetermined reasonable values the current that is likely to be drawn by a terminal from the transmission and power feed link to which it is connected. The addition of a switch mode power supply unit to a current limiter power feed system makes it possible to achieve greater flexibility in the use of the electrical power received by a terminal. However, a state in which the power feed system is disabled may arise if the power demand of the switch mode power supply unit exceeds what the power feed link can supply given the presence of the current limiter.
The existence of terminals fitted with standard connectors enables simple connection of a telephone and/or information technology installation according to the immediate and possibly temporary requirements of users, for example by simply plugging in at various points duly equipped with one or more connectors. It also implies that the terminals and in particular their power feed systems do not disrupt the operation of the remote power source, especially during connection operations, which can cause the power feed system to short-circuit the power source if the system comprises capacitors which are charged directly from the terminals of the remote power source via the transmission and power feed link to which the device is being connected.
It is also necessary to be able to connect a terminal to a transmission and power feed link already serving other terminals without interrupting their operation and any interchange of information that may be in progress. Failing perfect simultaneity of the connection of the various conductors of a terminal connector when making a connection to a multi-wire transmission and active power feed link, transient imbalance is likely to disrupt transmissions in progress on the link to an unacceptable degree.